With a clean block, MB label coolant and a 606 11 blade fan and clutch as recommended by speedy300turbo, my 93 SD 3.0 would stay under 90*C unless pushed really hard. Even at WOT in fourth up the Sunol and Altamont grades in the SF east bay it didn't cross 100*C. Ambient was never more than 90*F before its demise so I don't know how it would have fared idling on hot days. And I always have AC running.

What's the point? A 603 in a 140 is capable of running cooler unless there's something inherently hot about a 3.5. FWIW the 3.5 would idle close to 100*C on 100*F days but it had a dead clutch.

If you see 100*C while idling with the AC off, I'd be concerned.

Sixto
87 300D